Library /sys$common/syshlp/dbg$help.hlb DEBUG, Language Support, BASIC, Watchpoints  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    In BASIC, you can set a watchpoint only on variables that are
    declared in COMMON or MAP statements (static variables). You
    cannot set watchpoints on variables explicitly declared with the
    DECLARE statement.